System for enhancing the security of the e-mails transmitted from a sender (10) to a receiver (12) over a data transmission network such as Internet wherein a Message Transfer Agent (MTA) (14) associated with the sender is in charge of transmitting over the network an original e-mail sent by the sender. The MTA associated with the sender includes a message splitting means (16) adapted to divide the original e-mail into a plurality of chunks according to a predetermined algorithm and a predetermined list of relay MTAs (20, 22, 24) to which are forwarded the plurality of chunks. The system comprises a chunk assembly agent (28) for receiving from the relay MTAs the plurality of chunks and re-assembling them by using the predetermined algorithm in order to re-build the e-mail before sending it to the receiver.